The present invention relates to the field of radiologic imaging, which makes it possible to visualize an organ or organ part, usually of the human body.
In digital radiology, the digital image obtained depends, among other things, upon the dose of X-rays and the nature of the tissue of the organ examined. The organ examined may consist of tissues of various natures and the zone of interest of the organ examined may consist of a tissue of a given nature. This is the case in particular of the breasts, which are made up of adipose tissues and of glandular tissues.
On present-day mammography apparatus, an automatic exposure (AEC) cell is positioned behind what is presumed to be the glandular tissue. This positioning is done blindly.
It would thus be desirable to be able to automatically detect a tissue of determined nature, such as the glandular tissue of the breast and other tissues of the organ examined, in order to set the parameters of exposure as a function of the nature of the tissue to be examined.
The subject of the present invention thus is a method of automatic detection of a tissue of predetermined nature in an organ part examined by digital x-ray radiography and the use of said detection to establish the parameters of exposure of the organ part.
An embodiment of the method comprises the steps of:
a) establishing parameters of pre-exposure from a table of automatic optimization of parameters, a given theoretical composition and an estimated radiologic thickness of the organ part examined and for a low but sufficient dose of X-rays to obtain a pre-exposure image of sufficient quality for the differentiation of tissues of the organ part examined;
b) establishing a minimum dose level and of a maximum dose level from a predetermined mechanical thickness of the organ part examined and conversion of these dose levels into a minimum threshold signal level and a maximum signal threshold level, readable by a detector;
c) pre-exposing the organ part examined to obtain a pre-exposure image;
d) creation in a zone of interest of the field of the pre-exposure image of a multi-cellular image comprising columns in which each cell indexed with regard to a reference point represents the logarithmic mean of the signal levels between the minimum threshold level and the maximum threshold level of a predetermined number of pixels of the detector;
e) determination in each column of the position, with respect to the reference point, of the cell having the minimum signal level to obtain a set of retained cells;
f) selection from the set of retained cells of the one whose position is most distant from the reference point;
g) determination of a band, parallel to the reference point, including the retained cell, the width of which is proportional to the distance of the cell selected with respect to the reference point and search in this band for the cell having an absolute minimum signal level; and
h) using the absolute minimum signal level to establish the parameters of exposure of the organ part examined by X-rays.
The organ part examined is compressed by means of a compression device, such as a compression pad in the case of a breast, and it is preferable to ensure that this device is not located in the field of the image, or if it is there, to modify the mode of detection of the zone of interest.
To do this, after step (d), the following steps are taken:
(1) elimination of all cells not including enough pixels of a value between the minimum threshold level and the maximum threshold level;
(2) establishing a minimum position and a maximum position in depth, determined from the reference point, of a device which may perhaps be present for compression of the organ part examined;
(3) establishing a threshold value;
(4) establishing a line situated between the minimum and maximum positions in depth of any possible compression device and parallel to the reference point;
(5) comparing the threshold value of the variation in signal level between two consecutive cells along the line, and for all the cells on the line;
(6) recording two positions, if they exist, along the line for which variations in signal level have passed from a value greater than the threshold value to a value lower than the inverse of the threshold value;
(7) verifying the distance between the two recorded positions, when they exist, is greater than a predetermined value, satisfaction of the conditions of steps (6) and (7) determining the presence of the compression device; and
(8) carrying out of steps (e) to (h) in a zone delimited by the two positions recorded in step (6) and the minimum and maximum positions in depth established in step (2).